Offsets all layers by half. Image/Half_Tile_Offset Useful when creating a tilable texture. Just press Alt+I,O and you can start editing the image-edge seams.
A workflow-friendly fast alternative to going around every layer and selecting Layer/Transform/Offset... / Offset by x2,y2 / OK
Let's say we have this 256x256 crop:
After the Image/Half-Tile-Offset command, we get this:
There was no UI to get in the way of workflow, and you can switch around offsets at any time. (Creating tiled textures requres such switching back-and-forth)
After some quick edits for 2 minutes with the clone-stamp tool, we have this image:
To see how nicely it tiles, Filter/Map/Tile.../512/512/OK :